[SOLVED] OVMAPI_6091E Discover cluster failed. Worker server not found.

I tried to upgrade my OVM server form 3.4.1 to 3.4.2.

I got this error message in my OVM Manager screen:
 OVMAPI_6091E Discover cluster failed. Worker server not found.

I solved it by placing my OVM in mainanance mode myself.
I got this message in my test environment where there is only one VM server in the cluster.